Added release notes and upgrade guide for 0.38.0

docs/releases/RELEASE_0.38.0_HEADER.md

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+**`{{ .ProjectName }}` [0.36.0](https://github.com/mariadb-operator/mariadb-operator/releases/tag/0.36.0) is here!** 🦭
+
+We're thrilled to announce this new release packed with multiple enhancements contributed by our community members. A community-driven release like this one is the best way to celebrate that we are already **more than 50 contributors 🎉**:
+- https://github.com/mariadb-operator/mariadb-operator/graphs/contributors
+
+If you're upgrading from previous versions, don't miss the __[UPGRADE GUIDE](https://github.com/mariadb-operator/mariadb-operator/blob/main/docs/releases/UPGRADE_0.36.0.md)__ for a smooth transition.
+
+**Replication (alpha)**
+- [#967](https://github.com/mariadb-operator/mariadb-operator/pull/967)
+- [#974](https://github.com/mariadb-operator/mariadb-operator/pull/974)
+
+**Backup**
+- [#1008](https://github.com/mariadb-operator/mariadb-operator/pull/1008)
+- [#1013](https://github.com/mariadb-operator/mariadb-operator/pull/1013)
+
+**Services**
+- [#1009](https://github.com/mariadb-operator/mariadb-operator/pull/1009)
+
+**SQL**
+- [#1026](https://github.com/mariadb-operator/mariadb-operator/pull/1026)
+
+Huge thanks to our awesome contributors @hedgieinsocks, @onesolpark, @johanjk, and @P4sca1 for making this release possible! 🙇
+
+---
+
+We value your feedback! If you encounter any issues or have suggestions, please [open an issue on GitHub](https://github.com/mariadb-operator/mariadb-operator/issues/new/choose). Your input is crucial to improve `{{ .ProjectName }}`🦭.
+
+Join us on Slack: **[MariaDB Community Slack](https://r.mariadb.com/join-community-slack)**.

docs/releases/UPGRADE_0.38.0.md

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+# Upgrade guide 0.36.0
+
+This guide illustrates, step by step, how to migrate to `0.36.0` from previous versions. 
+
+> [!NOTE]  
+> Do not attempt to skip intermediate version upgrades. Upgrade progressively through each version.
+
+For example, if upgrading from `0.0.33` to `0.36.0`:
+An attempt to upgrade from `0.0.33` directly to `0.36.0` will result in unpredictable behavior.
+An attempt to upgrade from `0.0.33` to `0.34.0`, then `0.35.0`, and then `0.36.0` will result in success.
+
+- Uninstall you current `mariadb-operator` for preventing conflicts:
+```bash
+helm uninstall mariadb-operator
+```
+Alternatively, you may only downscale and delete the webhook configurations:
+```bash
+kubectl scale deployment mariadb-operator --replicas=0
+kubectl scale deployment mariadb-operator-webhook --replicas=0
+kubectl delete validatingwebhookconfiguration mariadb-operator-webhook
+kubectl delete mutatingwebhookconfiguration mariadb-operator-webhook
+```
+
+- Upgrade `mariadb-operator-crds` to `0.36.0`:
+
+```bash
+helm repo update mariadb-operator
+helm upgrade --install mariadb-operator-crds  mariadb-operator/mariadb-operator-crds --version 0.36.0
+```
+
+- If you are using Galera, and you want the operator to automatically update the data-plane (i.e. init and agent containers) to `0.36.0`, you can set `updateStrategy.autoUpdateDataPlane=true` in your `MariaDB` resources:
+
+```diff
+apiVersion: k8s.mariadb.com/v1alpha1
+kind: MariaDB
+metadata:
+  name: mariadb-galera
+spec:
+  updateStrategy:
++   autoUpdateDataPlane: true
+```
+
+Alternatively, you can also do this manually:
+
+```diff
+apiVersion: k8s.mariadb.com/v1alpha1
+kind: MariaDB
+metadata:
+  name: mariadb
+spec:
+  galera:
+    agent:
+-      image: docker-registry3.mariadb.com/mariadb-operator/mariadb-operator:0.35.0
++      image: docker-registry3.mariadb.com/mariadb-operator/mariadb-operator:0.36.0
+    initContainer:
+-      image: docker-registry3.mariadb.com/mariadb-operator/mariadb-operator:0.35.0
++      image: docker-registry3.mariadb.com/mariadb-operator/mariadb-operator:0.36.0
+```
+
+-  Upgrade `mariadb-operator` to `0.36.0`:
+```bash 
+helm repo update mariadb-operator
+helm upgrade --install mariadb-operator mariadb-operator/mariadb-operator --version 0.36.0 
+```
+
+- If you previously decided to downscale the operator, make sure you upscale it back:
+```bash
+kubectl scale deployment mariadb-operator --replicas=1
+kubectl scale deployment mariadb-operator-webhook --replicas=1
+```
+
+- If you previously set `updateStratety.autoUpdateDataPlane=true`, you may consider reverting the changes once the upgrades have finished:
+
+```diff
+apiVersion: k8s.mariadb.com/v1alpha1
+kind: MariaDB
+metadata:
+  name: mariadb-galera
+spec:
+  updateStrategy:
++   autoUpdateDataPlane: false
+-   autoUpdateDataPlane: true
+```